The role of ellipticity on the preferential binding site of Ce and La in C78-D3h--a density functional theory study

Endohedral metallofullerenes that encapsulate one or several atoms, or a cluster of atoms have molecular properties making them useful both in technology and in bio-medical applications. Some fullerenes are found to have two metal atoms incarcerated and it has been recently found that two Ce atoms are incorporated into the C(78)-D(3h) (78 : 5) cage. In this study, we report calculations on the structural and electronic properties of Ce(2)@C(78) using density functional theory (DFT). While Ce(2)@C(80)-I(h) (D(3d)) and La(2)@C(80)-I(h) (D(2h)) have different ground state structures, we have found that Ce(2)@C(78) has a D(3h) ground state structure just as La(2)@C(78). The encapsulated Ce atoms bind strongly to the C(78)-D(3h) cage with a binding energy (BE) of 5.925 eV but not as strong as in Ce@C(82)-C(2v) nor in Ce(2)@C(80)-I(h). The elliptical nature of the cage plays a crucial role and accommodates the two Ce atoms at opposite ends of the C(3) axis with a maximized inter atomic distance (4.078 A). This means that the effect of the additional f-electron repulsion in M(2)@C(78) with M = Ce compared to M = La, is less pronounced than in Ce(2)@C(80) compared to La(2)@C(80). We compare the results to the elliptical M(2)@C(72) (#10611) (M = La, Ce), and with a range of additional Ce and La endohedral fullerenes and explain the role ellipticity has in the preferential binding site of Ce and shed light on the formation mechanism of these nanostructures.